• How does it fit into the modern HVAC System?
  
• How do whole building system work?
  
• What does humidity control add to utility bills?
  
• How much do they cost to include / retrofit?
  

• In the HVAC engineering air is considered as being made up of only two components-Dry air and water vapor.
  
• The properties of air remain relatively unchanged as the temperature of the air rises and falls.
  
• The water vapor, on the other hand, may undergo considerable alteration as the temperature changes, including changes of state (condensing and freezing). Substantial amounts of energy are involved in these transformation.

• Describes the wetness or dryness of air at a given temperature and pressure.
  
• RH tells us the amount of moisture present in the air at a given temperature compared to what the air could hold at that temperature if it were saturated and is expressed as a percentage.
  
• If temperature rises 10F with moisture content remaining the same the RH goes down~2%.
  
• If the temperature drops 10F with the moisture content remaining the same the RH goes up ~2%.

• Each of these terms is expressed as number that describes a unit weight of water vapor associated with a unit weight of dry air. (grains or lbs of moisture/lb of dry Air)
  

Duct Equivalent Relative Humidity

• This is the relative humidity of a duct air stream at a given temperature as compared to the relative humidity of the space served which is usually at a different temperature

Dry Bulb Temperature

• The temperature of air indicated by any type of thermometer or thermocouple that has not been affected by evaporation or radiation.
  

Wet Bulb Temperature

• Expression of the temperature of the air when a wick or sock wetted with water encases the sensing element of a dry bulb thermometer and air is passed over it at a velocity of 700 ft per minute or more. The drier the air the greater is the cooling caused by evaporation and, therefore, the lower the wet bulb temperature.

Dew-Point Temperature

• The saturation temperature corresponding to the humidity ratio and pressure of a given moist-air state. It is the surface temperature at which moisture begins to condense on the surface. The more humid the air, the higher the due-point temperature. Conversely, the dryer the air, the lower the dew-point temperature. Air at 100 RH.

Vapor Migration

• In a mixture of water vapor and dry air, the water vapor exerts it own vapor pressure an will migrate from areas of higher vapor pressure to areas of lower vapor pressure. This migration occurs regardless of air movement. It is important to keep this phenomenon in mind when designing humidification for buildings or spaces within buildings. It may be necessary to consider the use of building materials having vapor barrier qualities in order to prevent loss of moisture, condensation and/or frost formation within the walls of the structure. Resulting in damage.

Latent Heat

• Latent means hidden. In HVAC usage, latent commonly refers to change of state, which is the heat involved in fusion (freezing water or melting ice) or vaporization (creating water vapor) condensation with no change in temperature. For water, fusion requires 144 btu per pound and vaporization and condensation requires 970 BTU per pound. These values, which are for sea-level atmospheric pressure, vary as pressure changes. Latent heat is not the same for all substances
  

Sensible Heat

• Sensible means that which can be sensed. In HVAC usage, it refers to the heat required to cause a change in temperature. The change is detected or sensed by the use of a thermometer

Air handling unit- Can be a single piece of equipment in small installations to field assembled components in large installations.

• Basic system includes Fan and Casing
  

Supplementary items-

• Cooling Coils to remove heat
  
• Heating Coils to add heat
  
• Filters to clean the air
  
• Control valves to regulate the amount of heat removed or added to system
  
• Dampers to control the flow of air
  
• Sensors used to supply feedback to the system controller
  
• Controller used to control set-points and sequences. Normally control valves and dampers in system. Can also provide information to other system controllers for monitoring and trending operation.
  

How Do Systems Work

Controlling Temperature-Heat removal

Function of removing the sensible heat dissipated in the space.

Sources-
Solar heat gain through glass, radiant heat from walls/roof heated by sun, people, lights, and outside air.

Each space’s temperature gain is dependent on its heat gain and the sensible cooling is provided by the volume of air and temperature difference below the set-point temperature.

Whole Building A/CHow Do Systems Work

Single Zone AHU

Basic System

Variable Air Volume Multiple Zone AHU

Each area has it's own thermostat for temperature control. Single AHU for cooling with supplemental heating on exterior zones.

Dual Duct-Constant Volume, Variable Volume

Basic requirements of HVAC Systems

• Supply a balanced volume of air at a temperature and moisture content to balance any heat gains or losses in the space and maintain the RH at the desired level.
  

Psychrometric Chart

• Added heat moves point to right
• Remove heat moves point left
• Added moisture moves point up.
  
• Remove moisture moves point down
• RH is curved lines up and to left
  
• Dewpoint horizontal
  

Basic Control Temperature- Sensible reading of a Thermometer

Add heat and temperature goes up. Remove heat and temperature goes down. Thermostat senses change and sends signal to add or remove heat.

RH Control Humidity-
Causes change in characteristic of Sensor

• Change in size
• Change in Capacitance
• Senses change in wet bulb sensor. (wet sock over thermometer sensing bulb).

Humidity Sensors Properties

Relative Humidity

• Changes with temperature with no change in moisture content.
• 1 Degree temperature change produces ~2% RH change.
  
• Changes with increased moisture content of air with no change in temperature.
  
• 1 Degree of Change in due point produces at constant temperature ~2% in RH.
  

Change of state

Vapor Migration

In a mixture of water vapor and dry air, the water vapor exerts it own vapor pressure an will migrate from areas of higher vapor pressure to areas of lower vapor pressure. This migration occurs regardless of air movement. It is important to keep this phenomenon in mind when designing humidification for buildings or spaces within buildings.

Air that mixes will seek a equilibrium in due point due point. If left untreated building air due point will be governed by the OA due point.

How does this effect hygroscopic materials

• Hygroscopic-Readily absorbing moisture, as from the atmosphere.
  
• Materials such as wood and building materials.
• Changes in weight.
  
• Changes in length and width.
  
• Changes in ability to support micro-organisms, grow mold, and deteriorate .

Equilibrium of Materials at different RH

Dimensional Change in Woods

Wood Equilibrium Moisture Content in Outdoors in several US Locations

Humidifier Technologies

Isothermic

• Electrode and Immersed Element
  
• Direct and Indirect Steam
  
• Gas Fired Steam

Adiabatic

• Air / Water Atomizer
  
• Airless Atomizer

"Isothermic"

• Internal Energy Exchange
• Heat added to water prior to being added to air stream
  
• Air temperature rise is due to heat loss of equipment and not in absorption of moisture.
  Isothermic Equipment

Isothermic Equipment

"Adiabatic"

• External Energy Exchange
  
• Energy for conversion of state from liquid to vapor taken from air stream. Air temperature drops as moisture is added at the rate of ~1000 BTUs/lb. of water
  
  

Adiabatic Equipment

Adiabatic Equipment

Droplet size critical to absorption. .3 to 10 micron available. Some equipment can provide only 20 micron and above. Not all can modulate output and are on or off.

How much moisture needs to be added?
(Simplified)

• Dominated by OA Conditions.
  
• Minimum is around 1 air change per hour.Unoccupied-25 CFM/Person Occupied
  
• Sq. ft of building times height=Volume in cubic ft
  
• Cubic ft/60 minutes for Cubic ft/min(CFM)
  
• Difference in moisture between Temp/RH of outdoor air and indoor desired condition.
  
• Multiply by CFM/100

How much moisture needs to be added?
Examples

• Building -100'X100'X14'=140,000 Cubic Ft.
  
• 140,000/60=2,333 CFM
  
• Outdoor Condition-20 Degrees F@ 70%--.758 lb/100 CFM
  
• Indoor Desired Condition-70 Degrees F@50%--3.44 lb/100 DFM
  

----------------------Occupied--------------------

• 3.44-.758=2.68 lb/100 X (2,333/100)=62 lbs/hr
  
• 280 peopleX25 CFM/person=7280 CFM Req.
  
• 7280/2333=3.12 3.12X62=193 lbs/hr

Cost for energy

• Per Hour
  
• Energy~1000 BTU/lb. of water X62=62,000 BTUH
  
• To
  
• 1000 BTU/lb. of water X193=193,000 BTUH
  
• Per Day (Times 24)-1,488,000 BTU to 4,632,000 BTU per day.435 kw to 1,356 kw/day
  
• $35 to $110 per day electric.
  

Added Cost

• Standby losses
  
• Maintenance
  
• Space allowance for equipment
  
• Replacement cost.

Installation Cost

• New Project Cost ~Institutional 100 year Building
  
• $150 to $225/ sq. ft Building not including land.
  
• Mechanical and Electrical Construction Cost 1/3 to ½ the cost With Humidification adding $1 to $2 Per Square Foot
  
• Self Contained in space equipment-$50/lb per hour capacity on 10 to 200 lb/hour equipment.